Titanium exhibits an extremely superior corrosion resistance in an atmospheric environment, so is being used for building material applications like roofing and walls in seashore regions. It has been more than a decade since titanium began to be used for roofing materials etc., but up until now there have been no examples reported of the occurrence of corrosion. Depending on the environment of use, however, sometimes the surface of the titanium used changes to a dark gold color over a long period of time. The discoloration is limited to the surface layer, so the anticorrosive function of the titanium is not impaired, but this is sometimes a problem from the viewpoint of the aesthetic appearance. To eliminate discoloration, the titanium surface has to be wiped with a mixed acid of nitric acid and fluoric acid, or another acid or else be lightly polished by polishing paper or a polishing agent to remove the discolored portion. When treating a large area of titanium on the surface such as with roofing, this is a problem from the viewpoint of the work efficiency.
The reasons for the occurrence of discoloration in titanium have still not been fully elucidated, but there are cases where it occurs due to Fe, C, SiO2, and the like floating in the air and depositing on the titanium surface and suggestions of the possibility of occurrence due to the increase in thickness of titanium oxide on the titanium surface. Further, as a method for lessening discoloration, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-1729, it has been reported to be effective to use titanium having an oxide film of not more than 100 angstroms on the titanium surface and reduced in surface carbon concentration to not more than 30 at %.
For the purpose of the prevention of discoloration, the inventors, however, conducted surface analyses of roofing materials made of titanium where discoloration had occurred at various parts of Japan and discoloration promotion tests to carefully study the effects of the thickness of the oxide film and surface carbon concentration on discoloration. As a result, they found that discoloration was not sufficiently prevented even by the invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-1729 and that there has not been any means up to now for fundamentally solving the problem of discoloration occurring in titanium used in an atmospheric environment.